The present invention relates to a storage system which constitutes a plurality of logical disks by combining a plurality of physical disks and, more particularly, to a storage system and data relocation method which increase the processing efficiency of each physical disk by relocating logical disks.
A general storage system is formed from a plurality of logical disks prepared by dividing data memory areas on a plurality of physical disks by an arbitrary unit and grouping the divided areas into an arbitrary number of sets. In the conventional storage system having this arrangement, if I/O (Input/Output) requests concentrate on a specific physical disk, a subsequent I/O request to the physical disk waits until the completion of the preceding I/O request, degrading the response. If this phenomenon occurs at part of the apparatus, this part bottlenecks the I/O processing performance of the overall apparatus.
A technique which solves this problem is disclosed in Japanese Patent Laid-Open No. 2000-215007 (reference 1). In a disk array apparatus disclosed in reference 1, the busy rate of a physical disk is measured, and if the measurement value exceeds a threshold designated by the user, a logical disk on the physical disk is selected. Data on the selected logical disk is transferred to another physical disk, and logical disk relocation of changing the correspondence between the logical disk and the physical disk is executed. As a result, concentration of I/O requests on some physical disks is canceled, and the I/O processing performance of the disk array apparatus is increased.
A similar technique is also disclosed in U.S. Pat. No. 6,061,761 (reference 2). In reference 2, the nonuniformity of the busy rate is minimized by exchanging logical disks. Logical disks to be exchanged are selected as follows. First, a list of pairs of exchangeable logical disks is created. Then, the variance of the busy rate of a physical disk after exchanging a pair of logical disks is calculated, and a pair of logical disks which provide a minimum variance is selected. This method can select a pair of exchangeable logical disks which minimize the nonuniformity of the busy rate. By repeating exchange, the busy rate of each physical disk is being leveled.
These techniques, however, may perform logical disk relocation leading to a decrease in I/O processing performance. The xe2x80x9cdecrease in I/O processing performancexe2x80x9d means an increase in the average of I/O request processing standby times on physical disks. This problem occurs in a storage system constructed such that the I/O processing performance values of physical disks differ from each other. The problem will be explained in detail for the two prior arts.
The technique disclosed in reference 1 executes logical disk relocation when the busy rate of a physical disk exceeds a threshold designated by the user. Reference 1 does not describe a criterion for the user to set the threshold of the busy rate. In other words, the user must determine a criterion used for logical disk relocation.
If the I/O processing performance is equal between physical disks, the user can set a predetermined threshold. However, when a storage system having different I/O processing performance values of physical disks is constructed, the user must set a threshold in accordance with the I/O processing performance of each disk. The user must set a threshold based on his/her criterion of judgment. The user may perform logical disk relocation which decreases the I/O processing performance.
The technique disclosed in reference 2 uses a parameter xe2x80x9cthe variance of the busy rate of a physical diskxe2x80x9d as the criterion of logical disk relocation. The variance of the busy rate of a physical disk after exchanging a pair of logical disks is calculated for each pair of exchangeable logical disks, and a pair of logical disks which minimize the variance is selected. This technique relocates logical disks so as to adjust the variance to xe2x80x9c0xe2x80x9d, i.e., level the busy rate of each physical disk.
However, a state in which the busy rate is leveled may not coincide with a state in which the average of I/O request processing standby times on physical disks is minimized. As long as physical disks have the same I/O processing performance, the state in which the busy rate is leveled must coincide with the state in which the average of I/O request processing standby times on physical disks is minimized.
When a storage system having different I/O processing performance values of physical disks is constructed, the state in which the busy rate is leveled does not coincide with the state in which the average of I/O request processing standby times on physical disks is minimized. For this reason, the technique disclosed in reference 2 may induce the apparatus to an unpreferable state. That is, logical disk relocation which decreases the I/O processing performance may be done.
It is an object of the present invention to provide a storage system and data relocation method capable of appropriately performing logical disk relocation.
To achieve the above object, according to the present invention, there is provided a storage system comprising a plurality of logical disks constituted by combining a plurality of physical disks, logical disk management means for converting input/output (I/O) requests addressed to the logical disks into I/O requests addressed to corresponding physical disks, and outputting the converted I/O requests, physical disk management means for accessing the corresponding physical disks in accordance with the I/O requests which are output from the logical disk management means and addressed to the physical disks, and logical disk relocation control means for relocating the logical disks so as to set busy rates of the physical disks within a range between a first state in which a ratio of the busy rates of the physical disks is equal to a ratio of I/O processing performance values of the physical disks, and a second state in which the busy rates of the physical disks are leveled.